In order to achieve a display which may be easily noticeable having a high contrast ratio in a wide viewing angle for a liquid crystal display device and the like, an optical film wherein refractive indexes within the surface of the optical film nx, ny, a thickness direction refractive index nz and a film thickness d are optimized has been used.
A phase difference layer with no phase difference or very little phase difference within the surface and having a phase difference only in a thickness direction is called a C-plate. C-plate is called a negative C-plate when its conditions of optical properties satisfy an inequality of nX≈nY>nZ, while it is called a positive C-plate when its conditions of optical properties satisfy an inequality of nZ>nX≈nY. Typical examples of the negative C-plate include a biaxially stretched polycarbonate (PC) or polyethylene terephthalate (PET) film, a film in which a selective reflection wavelength region of a cholesteric liquid crystal is set shorter than a visible ray, a film with a discotic liquid crystal oriented parallel to the surface, a film obtained by vertically aligning an inorganic crystalline compound having a negative birefringence and the like. Typical examples of the positive C-plate include vertically aligned liquid crystal films as disclosed, for example, in Patent Documents 1 and 2.
The phase difference within the surface of C-plate may be as small as possible from the viewpoint of maintenance of the polarization state of an incident light from the Z axis direction (normal direction) as it is, and it is preferably not more than λ/20, more preferably not more than λ/50 and ideally 0. However, since in the biaxially stretched polycarbonate (PC) or polyethylene terephthalate (PET) film, a photo-elastic coefficient was big (a phase difference caused by stretching orientation is easily exhibited), a big phase difference was exhibited due to a little stretching difference in the X and Y directions. Thus, it was difficult to diminish the phase difference within the surface. Furthermore, when a liquid crystal or an inorganic crystalline compound oriented to a base film or the like was used, it was possible to reduce the phase difference within the surface in some cases, whereas there was a problem of high cost because the number of materials and the number of processes were high.
Meanwhile, there have been disclosed a film using triacetyl cellulose, a cyclic olefin based polymer or the like in Patent Documents 3 and 4. In the production of these films, a so-called solution casting method including melting a resin in a solution to form a film has been used. For this reason, these films were also expensive because the number of materials and the number of processes were high, and sufficient considerations have been required for handling such films because a flammable solvent was used.
Patent Document 1: Japanese Patent Laid-open No. 2001-194668
Patent Document 2: Japanese Patent Laid-open No. 2004-118185
Patent Document 3: Japanese Patent Laid-open No. 2004-4905
Patent Document 4: Japanese Patent Laid-open No. 2004-177642